Many versions of photochromic glasses have been developed. Among all the attributes, the following are considered commercially important: low residual coloration in the clear state, color and transmission in the darkened state, ability to brighten reversibly (fading rate), and low thermal dependence, among others. In addition, some physical properties such as refractive index, coefficient of thermal expansion, Littleton temperature, and high temperature of crystallization are also desirable for manufacturing reasons.
French Patent Application 97/01164 describes improved photochromic glasses, for which composition range has been selected to optimize the above set of photochromic and physical attributes. These glasses are now commercially available, especially in countries where the weather is hot, offering attractive performances such as low thermal dependence and rapid fading. However, to obtain the required properties, these glasses include rare earth oxides (especially lanthanum oxide) to achieve a corrected refractive index without drastically impacting the photochromic properties.
Due to the recent huge increase of the cost of the raw materials and especially of those of the rare earth oxides, it is desirable to develop rare earth-free photochromic glasses presenting the same optical properties. In order to achieve this goal, rare earth oxides need to be replaced by other constituents.
It is to the provision of such technologies that the present disclosure is directed.